A third letter, similar to the ones sent to New York City Mayor Michael Bloomberg, that was addressed to President Barack Obama has been intercepted by the U.S. Secret Service and is being tested for ricin.

The letter never reached the White House and was intercepted at a mail-sorting facility set up after the anthrax attacks in 2001.

Two letters addressed to Bloomberg and his gun control advocacy group, Mayors Against Illegal Guns, tested positive for ricin earlier this week.

The letter sent to Obama contained identical text as missives sent to Bloomberg, NBC Washington reported, including the phrase, “what’s in this letter is nothing compared to what I’ve got planned for you.”

The Bloomberg letters were postmarked in Shreveport, La. The postal facility there handles mail from Louisiana, Texas and Arkansas, so the writer may be from any of those three states. Investigators haven't said whether they believe the writer is a man or a woman and haven't divulged if the letters were handwritten or typed. They also haven't said whether they believed all the letters were written by the same person.

Bloomberg said the threatening missives didn't faze him when he addressed the media Wednesday night as reports broke about the ricin letters.

"In terms of why they've done it, I don't know," the mayor said, according to the Associated Press. The letters "obviously referred to our anti-gun efforts, but there's 12,000 people that are going to get killed this year with guns and 19,000 that are going to commit suicide with guns, and we're not going to walk away from those efforts.”